3-Bit Multilevel Switching by Deep Reset Phenomenon in Pt/W/TaOX/Pt-ReRAM Devices

The influence of two different ohmic electrodes (W and Ta) on the resistive switching characteristics of TaO
_x
-based resistive random access memory (ReRAM) devices has been studied. Consistently, higher resistance OFF states have been observed with the W-ohmic electrode under the same operational conditions for all reset stop voltages (V
_reset-stop
) during both the dc and ac measurements. The deeper reset for samples with W-electrode is attributed to the less negative Gibbs-free energy of W-oxide compared with Ta-oxide, resulting in easier re-oxidation of the filament through oxygen exchange with the W-electrode. The higher R
_OFF
/R
_ON
(>10
³
) with the W-electrode enables 3-bit multilevel cell operation in the Pt/W/TaO
_x
/Pt ReRAM device. An excellent retention for these eight states is demonstrated at 125°C for 10
⁴
s. Furthermore, the TaO
_x
ReRAM device with both the electrodes shows high endurance up to 10
⁶
cycles based on two states.